The prior art has generally recognized a ten- to twenty-percent reduction in the gross weight capability of tiltrotor aircraft due to rotor-airframe flowfield interactions when the aircraft's engines are rotated into the hover position. More specifically, when the rotors are operated in the hover position, a portion of the rotor downwash from each wing-mounted rotor impinges upon the upper surface of the respective wing and is redirected spanwise along the wing's upper surface toward the mid-fuselage area separating the aircraft's wings. The inbound spanwise flows merge together to thereby increase static pressure above the mid-fuselage area and, hence, add a download force on the airframe. Moreover, a portion of the merged flow rises up above the airframe, only to be re-ingested by the rotors, thereby further reducing rotor efficiency while substantially increasing rotor noise. This latter effect is often referred to as the "fountain effect," with the recirculating flow being referred to as "fountain flow."
In response, the prior art teaches the use of wing-mounted "chordwise" structures, such as fences, plows, and slanted scoops, to redirect each inbound spanwise flow both forward and aft of the aircraft's mid-fuselage area. A typical characteristic of such prior art fences, plows, and scoops, is that a portion of the inbound spanwise flow stagnates at the base of the fence, plow or scoop. Such stagnated flow, in turn, generates increased static pressure along the wing's upper surface, thereby increasing download forces on the airframe.
Further, each prior art chordwise structure (typically deployed in pairs, one atop each wing) treats each spanwise flow in a "symmetrical" manner, i.e., one wing's chordwise structure defines a mirror-image of the other wing's chordwise structure about a vertical reference plane passing through a centerline axis of the fuselage. As a result, to the extent that the chordwise structures horizontally redirect each spanwise flow over the upper surfaces of the aircraft, the flows are redirected in a symmetrical manner fore and aft of the mid-fuselage area, thereby resulting in only a modest reduction in spanwise-flow-related download force.